1. Field of the Invention
The present invention generally relates to an optical sensing technique for discerning the types of objects or mediums based on their interaction with light. In particular, the present invention relates to an apparatus for discerning different kinds of recording sheets (plain paper, glossy paper, transparent sheets, etc.) from each other.
2. Description of the Related Art
Electrophotographic printers or copiers have been widely used for printing desired information on recording mediums, such as plain paper, glossy (or slick) paper and transparent plastic sheets (as used for overhead projectors or OHPs). To perform proper printing on these different types of recording sheets by an electrophotographic printer; it may be necessary to alter the print settings of the printer in accordance with the nature of the selected recording medium. For instance, to properly transfer toner-developed images from the photosensitive drum to a recording sheet, it may be necessary to increase or decrease the applied voltage, depending upon whether the recording sheet is a plain paper sheet or transparent plastic sheet (called xe2x80x9cOHP sheetxe2x80x9d below). Further, it may also be necessary to change the feeding speed of a recording sheet in passing through the fixing unit of the printer. Specifically, the feeding speed of an OHP sheet may need to be made slower than that of a plain paper sheet, so that the transferred toner images take sufficient time to melt properly, thereby obtaining great transparency.
To make efficient adjustments of the toner transfer voltage or sheet feeding speed described above, it is convenient if the types of the selected printing medium are automatically determined. Along this line, various kinds of automatic sensing techniques have been conventionally proposed.
For instance, JP-A-2 (1990)-56375 discloses a medium detecting device which utilizes an optical sensor. Specifically, as shown in FIG. 4 of the accompanying drawings, the conventional device is provided with an optical sensor 8 which includes one light-emitting unit 80 cooperating with two (or more) light-receiving units 81, 82.
The light-emitting unit 80 is arranged so that its center line makes a rather large angle xcex8(=60xc2x0xe2x88x9275xc2x0) with the normal line of the recording sheet S. The rays of light emitted from the unit 80 will partly be reflected regularly and partly be diffused at the point O on the recording sheet S. On the opposite side of the normal line, the first light-receiving unit 81 is arranged so that its center line makes the same angle xcex8 with the normal line. Thus, the rays of light regularly reflected on the sheet S will be received by the first light-receiving unit 81.
The second light-receiving unit 82 is disposed at a higher position and closer to the normal line than the first unit 81 is. Thus, the angle xcfx86 made between the normal line of sheet S and the center line of the unit 82 is smaller than the above-mentioned angle xcex8. With this arrangement, part of the light diffused at the point O on the sheet S will be received by the second unit 82.
The above conventional medium detecting device can distinguish one type of recording medium from another in the following manner.
When the recording medium S is plain paper, the light emitted from the unit 80 is diffused at the point O. Then, the diffused light will be received partly by the first light-receiving unit 81 and partly by the second light-receiving unit 82. Accordingly, both the first and the second units 81, 82 will output detection signals of similar or same strengths.
When the recording medium S is glossy paper, on the other hand, the light emitted from the unit 80 tends to be reflected regularly at the point O on the sheet S. Thus, most of the reflected light is received by the first unit 81, whereas only a small portion of the reflected light is received by the second unit 82. Accordingly, a strong detection signal will be outputted from the first unit 81, while a weak detection signal will be outputted from the second unit 82.
Thus, it is possible to determine whether the recording medium S is plain paper or glossy paper by monitoring the detection signals outputted from the first and the second light-receiving units 81, 82.
While the conventional device is functional in the above manner, it suffers from the following drawback.
As stated above, the conventional device can distinguish two types of recording paper (plain paper and glossy paper). However, since its distinction is based only on reflection light, it is difficult or substantially impossible for the conventional device to distinguish glossy paper and a transparent plastic sheet (used for e.g. an overhead projector) both of which have a slick surface, thereby regularly reflecting light from the unit 80 in the same manner. This means that the conventional device cannot make a correct distinction when the recording mediums to be used include three types of material such as plain paper, glossy paper and transparent sheets.
A second example of conventional medium detecting device is disclosed in JP-A-10(1998)-198174. The device includes one light-receiving unit for two or more light-emitting units. However, as in the above-described first conventional device, the second device can only distinguish two types of material such as plain paper and transparent OHP sheets, but cannot three types of material (plain paper, glossy paper and transparent sheets).
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention, to provide a sensing technique which makes it possible to distinguish at least three types of recording mediums such as plain paper, glossy paper and transparent sheets.
According to the present invention, there is provided an optical sensor which includes: a light-emitting element arranged on a first side of a transfer path along which an object is moved; a first light-receiving element arranged on a second side of the transfer path, the second side being opposite to the first side; a second light-receiving element arranged on the first side of the transfer path; and a third light-receiving element arranged on the first side of the transfer path.
Preferably, the transferred object maybe any one of plain paper, glossy paper and transparent plastic sheet.
Preferably, the light-emitting element may have a light output surface, while the first light-receiving element may have a light input surface facing the light output surface.
Preferably, the third light-receiving element may be spaced farther from the light-emitting element than the second light-receiving element is. In this case, the distance between the light-emitting element and the second light-receiving element may be smaller than the distance between the second and the third light-receiving elements.
Preferably, the light-emitting element may be disposed between the second and the third light-receiving elements.
Preferably, the light-emitting element, the second light-receiving element and the third light-receiving element may be disposed at substantially the same distance from the transfer path.
Preferably, each of the second and the third light-receiving elements may have a light input surface which is skew to the transfer path.
Preferably, the sensor of the present invention may further include a first discerning unit and a second discerning unit, wherein the first discerning unit is connected to the first light-receiving element, and the second discerning unit is connected to both the second and the third light-receiving elements.
Preferably, the second discerning unit may detect the difference in amount of received light between the second and the third light-receiving elements.
Preferably, the sensor of the present invention may further include a common casing which holds the light-emitting element and the second and the third light-receiving elements.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.